Very many chemical reactions of industrial importance, especially in the petroleum refining and petrochemical industry, involve the use of heterogeneous catalysis. Commonly, the catalysts used are heavy metals or transition metals such as chromium, molybdenum, tungsten, iron, titanium, vanadium, copper, cobalt, nickel, zirconium, niobium, tantalum and the like, or compounds thereof, and mixtures of one or more of these metals or compounds with other metals or compounds. Examples of such processes which depend on heterogeneous catalysis include selective oxidation of hydrocarbons, metathesis of olefins, isomerization of olefins, isomerization of alkanes, ring hydrogenation, olefin oligomerization, alkane and alkene dehydrogenation and the like.
The nature of the catalytic reaction and the chemical mechanism by means of which the heterogeneous catalytic material participates in and/or promotes the reaction has been the subject of intensive research studies in recent years. It has been accepted for many years that heterogeneous catalysis is a surface phenomenon, with the exposed surface of the catalyst apparently possessing active catalytic sites. Thus, the more finally divided a particulate heterogeneous catalyst, normally the higher its catalytic activity. More recently, it has been determined that the most catalytically active form of a metal in a supported or unsupported heterogeneous catalyst, in at least 95% of cases, is a small-cluster form of the metal with up to about 15 metal atoms formed into a "cluster". This is perhaps the ultimate extension of fine subdivision of the heterogeneous catalyst.
The properties of the metal when in the form of small clusters of a few atoms only, are different from those of the metal when in stable, bulk form. This is believed due at least in part to the fact that metal atoms in small clusters are separated from other atoms of the metal to a sufficient extent that the interaction effects therewith are neglible. In bulk metal, in contrast, each individual atom of the metal is in fixed spatial relationship and close proximity to very large numbers of other, similar atoms of the same metal, and the resulting interactions have profound effects on the properties of the bulk metal.
Small cluster forms of metal are, however, extremely short lived and unstable under normal conditions. Whilst metals may be generated in uni-atomic condition, especially in the vapour phase by suitable evaporation, the atoms will very rapidly agglomerate together, firstly into small clusters but then growing into bulk material. The prevention of this agglomeration, to produce metal in small cluster form, poses problems. It is particularly difficult to produce small-cluster form of metal which is stable at or close to room temperatures.